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Public Key Infrastructure (X509 PKI)

Public Key Infrastructure (X509 PKI)

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Public Key Infrastructure (X509 PKI)

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  1. Public Key Infrastructure (X509 PKI) Marco Casassa Mont Trusted E-Services Laboratory - HP Labs - Bristol

  2. Outline • Basic Problem of Confidence and Trust • Background: Cryptography, Digital Signature, Digital Certificates • (X509) Public Key Infrastructure (PKI) • (X509) PKI: Trust and Legal Issues

  3. Confidence and Trust Issues in the Digital World

  4. Intranet Extranet Internet Bob Alice Basic Problem Bob and Alice want to exchange data in a digital world. There are Confidence and Trust Issues …

  5. Intranet Extranet Internet Alice Bob Confidence and Trust Issues • In the Identity of an Individual or Application AUTHENTICATION • That the information will be kept Private CONFIDENTIALITY • That information cannot be Manipulated INTEGRITY • That information cannot be Disowned NON-REPUDIATION

  6. Starting Point: Cryptography

  7. Starting Point: Cryptography Cryptography It is the science of making the cost of acquiring or altering data greater than the potential value gained Cryptosystem It is a system that provides techniques for mangling a message into an apparently intelligible form and than recovering it from the mangled form Plaintext Encryption Ciphertext Decryption Plaintext &$*£(“!273 Hello World Hello World Key Key

  8. Cryptographic Algorithms All cryptosystems are based only on three Cryptographic Algorithms: MESSAGE DIGEST (MD2-4-5, SHA, SHA-1, …) Maps variable length plaintext into fixed length ciphertext No key usage, computationally infeasible to recover the plaintext SECRET KEY (Blowfish, DES, IDEA, RC2-4-5, Triple-DES, …) Encrypt and decrypt messages by using the same Secret Key PUBLIC KEY (DSA, RSA, …) Encrypt and decrypt messages by using two different Keys: Public Key, Private Key (coupled together)

  9. Plaintext Encryption Ciphertext Decryption Plaintext Private Key Private Key Cryptographic Algorithms based on Private Key Pros • Efficient and fast Algorithm • Simple model  Provides Integrity, Confidentiality Cons • The same secret key must be shared by all the entities involved in the data exchange • High risk • It doesn’t scale(proliferation of secrets)  No Authentication,Non-Repudiation

  10. Plaintext Encryption Ciphertext Decryption Plaintext Intranet Extranet Internet Alice’s Private Key Alice’s Public Key Alice Bob Cryptographic Algorithms based on Public Key Pros • Private key is only known by the owner: less risk • The algorithm ensures Integrity and Confidentiality by encrypting with the Receiver’s Public key

  11. Plaintext Encryption Ciphertext Decryption Plaintext Intranet Extranet Internet Bob’s Public Key Bob’s Private Key Alice Bob Cryptographic Algorithms based on Public Key Pros • The algorithm ensures Non-Repudiation by encrypting with the Sender’s Private key

  12. Intranet Extranet Internet Alice Bob Cryptographic Algorithms based on Public Key Cons • Algorithms are 100 – 1000 times slower than secret key ones They are initially used in an initial phase of communication and then secrets keys are generated to deal with encryptions • How are Public keys made available to the other people? • There is still a problem of Authentication!!! Who ensures that the owner of a key pair is really the person whose real life name is “Alice”? Moving towards PKI …

  13. Digital Signature

  14. Digital Signature A Digital Signature is a data item that vouches the origin and the integrity of a Message • The originator of a message uses a signing key (Private Key) to sign the message and send the message and its digital signature to a recipient • The recipient uses a verification key (Public Key) to verify the origin of the message and that it has not been tampered with while in transit Intranet Extranet Internet Alice Bob

  15. Digital Signature Message Message Digest Algorithm Digest Algorithm Hash Function Hash Function Digest Public Key Encryption Decryption Private Key Expected Digest Actual Digest Signature Signer Receiver Channel

  16. Digital Signature There is still a problem linked to the “Real Identity” of the Signer. Why should I trust what the Sender claims to be? Moving towards PKI …

  17. Digital Certificate

  18. Digital Certificate A Digital Certificate is a binding between an entity’s Public Key and one or more Attributes relating its Identity. The entity can be a Person, an Hardware Component, a Service, etc. A Digital Certificate is issued (and signed) by someone - Usually the issuer is a Trusted Third Party A self-signed certificate usually is not very trustworthy

  19. CERTIFICATE Digital Certificate Issuer Subject Subject Public Key Issuer Digital Signature

  20. Digital Certificate Problems • How are Digital Certificates Issued? • Who is issuing them? • Why should I Trust the Certificate Issuer? • How can I check if a Certificate is valid? • How can I revoke a Certificate? • Who is revoking Certificates? Moving towards PKI …

  21. Public Key Infrastructure (PKI)

  22. Public Key Infrastructure (PKI) A Public Key Infrastructure is an Infrastructure to support and manage Public Key-based Digital Certificates

  23. Public Key Infrastructure (PKI) “A PKI is a set of agreed-upon standards, Certification Authorities (CA), structure between multiple CAs, methods to discover and validate Certification Paths, Operational Protocols, Management Protocols, Interoperable Tools and supporting Legislation” “Digital Certificates” book – Jalal Feghhi, Jalil Feghhi, Peter Williams

  24. Public Key Infrastructure (PKI) Focus on: • X509 PKI • X509 Digital Certificates  Standards defined by IETF, PKIX WG: … even if X509 is not the only approach (e.g. SPKI)

  25. X509 PKI – Technical View Basic Components: • Certificate Authority (CA) • Registration Authority (RA) • Certificate Distribution System • PKI enabled applications “Provider” Side “Consumer” Side

  26. X509 PKI – Simple Model Certification Entity CA Cert. Request RA Application Service Signed Certificate Internet Certs, CRLs Directory Remote Person Local Person

  27. X509 PKI Certificate Authority (CA) Basic Tasks: • Key Generation • Digital Certificate Generation • Certificate Issuance and Distribution • Revocation • Key Backup and Recovery System • Cross-Certification

  28. X509 PKI Registration Authority (RA) Basic Tasks: • Registration of Certificate Information • Face-to-Face Registration • Remote Registration • Automatic Registration • Revocation

  29. X509 PKI Certificate Distribution System Provide Repository for: • Digital Certificates • Certificate Revocation Lists (CRLs) Typically: • Special Purposes Databases • LDAP directories

  30. Certificate Revocation List Certificate Revocation List Revoked Certificates remain in CRL until they expire

  31. Certificate Revocation List (CRL) • CRLs are published by CAs at well defined interval of time • It is a responsibility of “Users” of certificates to “download” a CRL and verify if a certificate has been revoked • User application must deal with the revocation processes

  32. Online Certificate Status Protocol (OCSP) • An alternative to CRLs • IETF/PKIX standard for a real-time check if a certificate has been revoked/suspended • Requires a high availability OCSP Server

  33. CRL vs OCSP Server Download CRL CRL User CA CRL Directory Certificate IDs to be checked Download CRL CRL User OCSP Server CA Answer about Certificate States Directory OCSP

  34. X509 PKI PKI-enabled Applications Functionality Required: • Cryptographic functionality • Secure storage of Personal Information • Digital Certificate Handling • Directory Access • Communication Facilities

  35. X509 PKI Trust and Legal Issues

  36. X509 PKI Trust and Legal Issues • Why should I Trust a CA? • How can I determine the liability of a CA?

  37. X509 PKI Approaches to Trust and Legal Aspects • Why should I Trust a CA? • How can I determine the liability of a CA? Certificate Hierarchies, Cross-Certification Certificate Policies (CP) and Certificate Policy Statement (CPS)

  38. X509 PKI Approach to Trust Certificate Hierarchies and Cross-Certification

  39. Directory Services LRA CA CA CA RA RA RA CA CA RA CA CA RA RA CA LRA Internet Internet CA Technology Evolution Try to reflect Real world Trust Models

  40. Simple Certificate Hierarchy Root CA Each entity has its own certificate (and may have more than one). The root CA’s certificate is self signed and each sub-CA is signed by its parent CA. Each CA may also issue CRLs. In particular the lowest level CAs issue CRLs frequently. End entities need to “find” a certificate path to a CA that they trust. Sub-CAs End Entities

  41. * Alice Bob Simple Certificate Path Trusted Root Alice trusts the root CA Bob sends a message to Alice Alice needs Bob’s certificate, the certificate of the CA that signed Bob’s certificate, and so on up to the root CA’s self signed certificate. Alice also needs each CRL for each CA. Only then can Alice verify that Bob’s certificate is valid and trusted and so verify the Bob’s signature.

  42. Cross-Certification and Multiple Hierarchies 1 2 3 • Multiple Roots • Simple cross-certificate • Complex cross-certificate

  43. X509 PKI Approach to Trust : Problems Things are getting more and more complex if Hierarchies and Cross-Certifications are used

  44. Cross-Certification and Path Discovery Trusted Root Trusted Root 3 *

  45. X509 PKI Approach to Legal Aspects Certificate Policy And Certificate Practice Statement

  46. Certificate Policy (CP) • A document that sets out the rights, duties and obligations of each party in a Public Key Infrastructure • The Certificate Policy (CP) is a document which usually has legal effect • A CP is usually publicly exposed by CAs, for example on a Web Site (VeriSign, etc.)


  48. Policy Issues (CP) Liability Issues • Repository Access Controls • Confidentiality Requirements • Registration Procedures - Uniqueness of Names - Authentication of Users/Organisations Certificate Acceptance • Suspension and Revocation (Online/CRL) • Physical Security Controls

  49. Certificate Policy Statement (CPS) • A document that sets out what happens in practice to support the policy statements made in the CP in a PKI • The Certificate Practice Statement (CPS) is a document which may have legal effect in limited circumstances